Waste heat is often created as a byproduct of industrial processes where flowing streams of high-temperature liquids, gases, or fluids must be exhausted into the environment or removed in some way in an effort to maintain the operating temperatures of the industrial process equipment. Some industrial processes utilize heat exchanger devices to capture and recycle waste heat back into the process via other process streams. However, the capturing and recycling of waste heat is generally infeasible by industrial processes that utilize high temperatures or have insufficient mass flow or other unfavorable conditions.
Waste heat can be converted into useful energy by a variety of turbine generator or heat engine systems that employ thermodynamic methods, such as Rankine and Brayton cycles. Rankine cycles, Brayton cycles, and similar thermodynamic methods are typically steam-based processes that recover and utilize waste heat to generate steam for driving a turbine, turbo, or other expander connected to an electric generator, a pump, or other device.
An organic Rankine cycle utilizes a lower boiling-point working fluid, instead of water, during a traditional Rankine cycle. Exemplary lower boiling-point working fluids include hydrocarbons, such as light hydrocarbons (e.g., propane or butane) and halogenated hydrocarbon, such as hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) (e.g., R245fa). More recently, in view of issues such as thermal instability, toxicity, flammability, and production cost of the lower boiling-point working fluids, some thermodynamic cycles have been modified to circulate non-hydrocarbon working fluids, such as ammonia.
Typically, in a heat engine system converting waste heat into useful energy, heated working fluid utilized therein is expanded in an expansion device, and the expansion device may convert the thermal energy into mechanical energy. The expanded working fluid may be cooled in a condenser before entering a main compressor of the heat engine system. Those of skill in the art will appreciate that the pressure of the working fluid at the inlet of the main compressor may affect the performance and operation of the heat engine system. Accordingly, one such approach to control the pressure of the working fluid at the inlet of the main compressor provides for the use of a pump and a storage tank including additional working fluid. The additional working fluid from the storage tank may be supplied to the heat engine system via the pump to increase the pressure of the working fluid at the inlet of the main compressor as needed. However, such an approach, while effective, may be impractical based on the allotted space for the heat engine system and the required size of the storage tank to contain enough additional working fluid to adequately control the pressure of the working fluid at the inlet of the main compressor. Further, such an approach requires a high head, high flowrate pump, which increases the complexity and time required to start up and also the operating costs and maintenance of the heat engine system.
Therefore, there is a need for a system and method for controlling the pressure of the working fluid at the inlet of the main compressor or pump of the heat engine system which reduces the footprint of the heat engine system and maximizes the efficiency of transforming thermal energy to mechanical and/or electrical energy.